I don't know of a way to re-size a Checkbox. However, you can always use a
Textbox. The Textbox can be re-sized and you can associate a macro with it
to insert/remove the checkmark.
This also gives you the ability to format the checkmark as well.
You might be able to set your font to a smaller size then increase the zoom.
Increasing the zoom will bring your font back to its orginal size (or close
to it...experiment!) and at the same time increase the size of the checkbox.